<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script src="js/jquery-3.4.1.min.js"></script>

    <button>测试</button> 
    <ul>
        <li>a</li> 
        <li>b</li> 
        <li>c</li> 
    </ul>

    <script>
       /* 后代是子、孙、曾孙等等
            children( [selector] ) 方法返回被选元素的所有直接子元素，“孩子” */
       $("button").click(function(){
           var x = $("ul").children().text(); //所有子节点:abc
           var x = $("ul").children("li:first").text(); //ul中的第一个子节点，a
           alert(x);
       });
    </script>
</body>
</html>